Gratings are optical devices used to achieve wavelength-dependent characteristics by means of optical interference effects. These wavelength-dependent optical characteristics can, for instance, serve to reflect light of a specific wavelength while transmitting or refracting light at all other wavelengths. Such characteristics are useful in a wide range of situations, including the extraction of individual wavelength-channels in Wavelength Division Multiplexed (WDM) optical communication systems, or providing wavelength-specific feedback for tunable or multi-wavelength semiconductor lasers. Gratings are usually implemented by modulating (varying) the effective index of refraction of a wave-guiding structure. These changes in index of refraction cause incident light wavelengths to be reflected or refracted: in the case of an abrupt interface between two index values, light incident directly on the interface is reflected according to the well-known Fresnel reflection law.
The term “multi-wavelength grating” generally refers to a grating that is capable of exhibiting optical characteristics at a number of wavelengths. For example, a multi-wavelength grating may be a grating that reflects light at several select wavelengths (which may correspond to specific optical communication channels), yet is transparent to light at other wavelengths. In some situations, however, there is a need to set the optical characteristics for a continuous range of wavelengths, rather than at specific wavelength values; for example, when using an optical grating to compensate for the unevenness of optical gain profiles in laser cavities and optical amplifiers. However, achieving this requirement for a continuous range of wavelengths is difficult to meet with traditional grating technologies.
Similarly, a range of optical wavelengths may be used where many communication channels are encoded into a single optical cable by utilizing different wavelengths of light; this is more commonly known as Wavelength Division Multiplexing (WDM) technology. Periodic gratings are often used to separate or process these channels. However, periodic grating technologies process one wavelength, forcing devices intended to process multiple wavelengths to employ multiple single-wavelength periodic gratings. This is not an attractive solution because, on top of the additional losses that each grating creates, even a single grating occupies a considerable amount of space by today's standards of integration and miniaturization. It is thus desired to have a single device capable of processing several wavelengths in a space-efficient manner.
In the field of semiconductor lasers, the output wavelength of semiconductor lasers is largely determined by the presence of “feedback elements” around or inside the laser gain section, which act to reflect light at the desired wavelength back into the laser. For multi-wavelength operation, multi-wavelength feedback is needed. Again, single-wavelength grating technology can only address this demand with a cascade of simple gratings, leading to the same (if not more notable) loss and space problems mentioned above.
In the field of optical transmission, it is well known that optical networks must contend with a property known as dispersion. This property arises from the wavelength-dependence of effective index, which in turn produces a wavelength-dependent group delay spectrum for a given type and length of optical fiber. Since an optical pulse always possesses some spectral width, this wavelength-dependence leads to different retardation of various spectral components of the optical pulse, thereby leading to its spread in the spatial domain. This spread directly impedes the operation of the optical network. Some forms of dispersion can be corrected for with single-wavelength gratings, but only on a channel by channel basis. More complicated forms of dispersion such as dispersion slope cannot be suitably corrected by single-wavelength gratings at all.
One such single-wavelength grating device is a Bragg Grating. The Bragg Grating consists of a periodic variation in refractive index and acts as a reflector for a single wavelength of light related to the periodicity (known as pitch, Λ) of the index pattern; and is frequently used in both semiconductor systems and fiber-optic systems. In practice, the Bragg Grating can usually reflect at several wavelengths, corresponding to overtones of its fundamental pitch; however, these higher-order wavelengths tend to be at quite different spectral regions than that of the fundamental wavelength, thus not making the Bragg Grating useful as a multi-wavelength reflector. Moreover, these higher-order wavelengths cannot be tuned independently of one another.
Other multi-wavelength grating technologies include: analog superimposed gratings, Sampled Gratings (SG), Super-Structure Gratings (SSG), Chirped Bragg Gratings, Dammann Gratings, Arrayed Waveguide Gratings (AWG), Echelle Gratings and Binary Superimposed Gratings (BSG).
Analog superimposed gratings are a generalization of the Bragg Grating and are rooted in a principle of superposition: a grating profile consisting of the sum of the index profiles of single-wavelength gratings reflects at all of its constituent wavelengths. Such a grating relies on an analog index variation, that is, a refractive index that changes continuously along the grating length. However, it is difficult to inscribe strong analog gratings using the well-known photorefractive effect, since the change of index under illumination varies non-linearly with stronger exposures, making the writing process difficult in semiconductors where surface relief gratings are used. It is also very difficult and generally impractical to reproducibly etch analog features into the surface of the semiconductor. The latter difficulty brought about the introduction of binary gratings, i.e., gratings that rely only on two refractive index values corresponding to the material being etched or not etched, illuminated or not illuminated.
Two representations of multi-wavelength binary gratings are sampled gratings (SG) and superstructure gratings (SSG). The SG is constructed with alternating sections of grating and grating-free regions of the waveguide. The alternating sections produce diffraction spectra consisting of multiple reflectance peaks contained within a (typically) symmetric envelope. The SG is intrinsically limited in the flexibility in the location and relative strength of reflectance peaks, and, because of the large fraction of grating-free space, is also spatially inefficient. The SG is therefore particularly unsuitable where a short grating is required or where waveguide losses are high.
With the super-structure grating (SSG), the grating period is chirped by finely varying the grating pitch, which corresponds to the length of one tooth-groove cycle. This can also be thought of as a sequence of finely tuned phase shifts; common phase profiles include linear and quadratic chirp. Such an implementation in principle allows arbitrary peak positions and relative heights, but only at the expense of extremely high resolution, corresponding to a very small fraction of the size of the grating teeth themselves.
Chirped Bragg Gratings are grating devices targeted at applications such as dispersion compensation and optical pulse compression. Here a Bragg grating's pitch L is varied along its length. This produces a wavelength-dependent phase spectrum which can be tailored to provide the desired group delay spectrum: τg=−dφ/dω. The delay for a given free-space wavelength λ0 then follows from the round-trip distance to where local pitch has λ0 as its Bragg wavelength: τg(λ0)=2neffz(λ0), where z(λ0) is the spatial coordinate at which Λ(z)=λ0/2neff. In practice, however, these implementations suffer from excessive group-delay ripple, indicating that the dispersion compensation is not complete.
Dammann Gratings are binary gratings devices wherein the grating features are imposed on some surface and wherein the incident light illuminates the surface at some normal or off-normal angle. The optical wavefront incident on this grating experiences a one-time interaction with the grating features and thereby experiences Raman-Nath type diffraction (as opposed to Bragg diffraction). This device is intended for free-space use and is not easily employed in guided-wave applications. Furthermore, to achieve the wavelength resolution requirements imposed by modern optical communication systems the incident light must be collimated to a very high degree, which can prove difficult in practice.
Arrayed Waveguide Gratings (AWG) are used primarily to spatially separate optical channels in a WDM environment. They operated by dividing input multi-wavelength light between an array of waveguides, wherein each waveguide is of a slightly different optical length. The resulting optical phase differences between the waveguides' respective outputs leads to a wavelength-dependent interference pattern, which with proper design can lead to a separation of wavelength components. In practice, this technology requires vast amounts of semiconductor real estate and imposes extreme manufacturing constraints.
Echelle gratings are also used primarily to spatially separate optical channels in a WDM environment. Here, a grating plane is generated by means of defining sub-wavelength reflective features at various glazing angles and potentially along some curved plane. The grating plane is then illuminated with multi-wavelength light, often at an oblique angle, and the individual reflections add up to substantially separate the wavelength components. The device tends to be very difficult to implement in practice, requiring both deep and flat etching characteristics when implemented in semiconductor.
Prior art regarding binary superimposed grating synthesis is presented in Ivan A. Avrutsky, Dave S. Ellis, Alex Tager, Hanan Anis, and J. M. Xu, “Design of widely tunable semiconductor lasers and the concept of Binary Superimposed Gratings (BSG's),” IEEE J. Quantum Electron., vol. 34, pp. 729-740, 1998.
Older methods in the prior art address the synthesis of “multi-peak” gratings—i.e., gratings characterized by reflectance at several “peaks”, which can be controlled in their position and strength. In these methods, a grating engineer begins with a set of sinusoids, each sinusoid corresponding to a single reflectance peak and weighted according to that peak's desired relative strength. These peaks are added together (i.e. superimposed; hence the BSG is known as a superimposed grating) to produce an “analog profile”. This profile is then digitally quantized by a simple threshold method. For example, if the analog profile value is positive (above a pre-selected reference) then the corresponding BSG segment is a high or binary 1 index value; if it is negative, the corresponding BSG segment is a low or binary zero index value.
However, this approach is inadequate in at least two areas: firstly, the threshold quantization process introduces intermodulation, which largely limits the applicability of BSGs synthesized in this manner to active applications (laser feedback elements and the like). Secondly, this synthesis procedure is limited to multi-peak gratings, and offers little or no control over the individual peak shape. It is also entirely incapable of generating flat-top channels, as desired by some communication applications, and of generating the near-arbitrary reflectance spectra demanded by some gain- and dispersion-compensation schemes.
Other methods for BSG synthesis include trial-and-error methods that are most often computationally difficult and inefficient.
Therefore, for detecting optical wavelengths in optical devices it is desirable to provide methods and apparatuses for overcoming the disadvantages noted above.